It is known to provide a receiver for an optical transmission system which converts an optical signal to electrical form, then samples the signal to detect digital ones and zeroes for example. Such sampling requires an amplitude threshold to be set, and a clock phase to be set. This usually involves providing a clock recovery circuit for determining and continually adjusting the timing of the sampling. Such clock recovery circuits typically use a phase locked loop controlled in association with the threshold level to maximize an “eye opening” of the received waveform.
There are various ways of modulating the data at the transmitter and demodulating at the receiver, most based on altering phase or amplitude. Quadriphase or quadrature phase shift keying (QPSK) is a technique of phase modulating digital information onto a carrier signal. QPSK communications systems are generally known in the art. In these systems, a transmission unit uses a local reference oscillator to generate a carrier wave. This oscillator in the transmitter determines the frequency and phase of the unmodulated carrier wave. The transmitter encodes two bits of digital information on the carrier by shifting the carrier phase by a multiple of 90° for an interval of time of length T. The four possible phase shifts, or symbols, that can be transmitted during this interval are determined by the four possible combinations of the two bits to be transmitted. One symbol (two bits of information) is transmitted during each time interval, so the rate of transfer of data in the system is two bits per symbol interval T. On the other end of this system, a receiving unit decodes the two transmitted bits by measuring the phase shift between the received signal and a local reference oscillator.
A drawback to this system is the requirement that the receiver must have a reference oscillator matched in phase to the transmitter oscillator; that is, the decoding is done by coherent detection. This requirement is relaxed in the technique of differential QPSK (DQPSK). In DQPSK, the transmitted data are differentially encoded, that is, they are represented by the difference in phase between successive symbols. In this technique, the receiver does not need the absolute phase of the transmitter oscillator to decode the transmitted symbols. Instead, the decoding of the symbols is by differentially coherent detection: the receiver measures the phase difference between two successive received symbols. This measurement yields a number with four possible values (0 degrees, 90 degrees, 180 degrees, 270 degrees) that represents the two bits of transmitted data.
To demodulate the received signal, it is conventionally desirable for the DQPSK receiver to have a local oscillator that reconstructs the carrier wave. This oscillator in the receiver must match the frequency (although not necessarily the phase) of the DQPSK transmitter oscillator that generated the carrier wave. If the frequencies of the two oscillators are not matched, the receiver cannot efficiently demodulate the transmitted data. The receiver oscillator can be built so that its natural frequency is close to that of the transmitter, but due to variations in manufacturing and differences in operating environments, there will be drifts between the two oscillators. To compensate for such offsets in frequency between the carrier wave and the receiver oscillator, the receiver oscillator can be locked to the carrier wave by a phase-locked loop (PLL). Such a carrier-recovery mechanism can serve to tie the frequency of the receiver oscillator to the frequency of the transmitter oscillator.
This carrier-recovery mechanism must be able to ignore variations in the carrier phase that are due to the information encoded into the carrier. That is, changes in the phase of the carrier by multiples of 90 degrees must not be interpreted as a drift in the receiver oscillator's frequency. U.S. Pat. No. 6,097,768 shows a phase detector for carrier recovery in a DQPSK receiver. It shows using simple arithmetic operations to measure phase errors in the carrier-recovery mechanism for a DQPSK digital communications receiver. The carrier-recovery mechanism is a feedback loop that provides a synchronization between the oscillators in the transmitter and receiver of the communications system. The phase detector measures deviations from this synchronization and generates a phase-error signal used in the feedback loop to synchronize the oscillators. To perform this measurement, the phase detector takes the received signal as input and compares it against a local oscillator in the receiver to generate two digital signals: the in-phase (I) and quadrature-phase (Q) components of the received signal. These signals are the input to a logic unit, which uses these two signals to determine the phase-error signal. In one embodiment of the phase detector, the logic unit analyzes the signs of the two digital signals and then accordingly adds or subtracts the I and Q signals to generate the phase-error signal. In another embodiment, the logic unit determines the magnitude of the phase-error signal by finding the difference in magnitudes of the two digital signals and constructing a phase-error signal proportional to this difference. The logic unit then determines the sign of the phase-error signal by analyzing the signs of the I and Q digital signals. The logic unit thus uses simple arithmetic operations to generate the phase-error signal, thereby reducing the complexity and cost of the phase detector.
In an optical DQPSK system it is possible to avoid regenerating a carrier in the receiver. United States Patent Application 20040141222 shows an optical phase multi-level modulation method and apparatus, and error control method. It uses a plurality of phase modulators disposed in series to phase-modulate light from a source laser. By using two phase-modulators connected in series to effect phase modulation, the frequency bandwidth required by the phase modulators is half that required in the case of phase modulation using a single phase modulator, so the phase modulator configuration can be simplified. At the receiver a demodulator splits the received lightwave along two pairs of optical paths, a one-bit delay is imparted to the light on a first path, and light on a second path is phase-shifted 45 degrees, and delay detection is effected by combining the light of these two paths. The same is done to the third and fourth paths, but there is a phase difference of 90 degrees between the phase-shift amounts imparted by the phase-shifters. Next, the light is converted to an electric signal by a balanced detector to demodulate the I- and Q-component signals. The absolute amount of the phase-shift is an arbitrary value and should be set from the standpoint of convenience and simplicity of the system apparatus.
United States Patent Application 20040081470 shows an optical communications method of transmitting a plurality n data streams comprising modulating an optical carrier using differential M-ary phase shift key (DMPSK) signaling in which M=2n. Advantageously the method comprises using differential quaternary phase shift keying in which n=2. Since the data is differentially encoded in the form of phase changes rather than absolute phase values this enables the modulated optical carrier to be demodulated using direct detection without requiring a phase-locked local optical oscillator. Optical DQPSK is said to provide a higher tolerance to chromatic dispersion and a higher tolerance to polarization mode dispersion, and the electrical and optoelectronic components operate with a bandwidth commensurate with half the line bit rate. Compared to coherent QPSK, optical DQPSK provides improved tolerance to cross-phase modulation (XPM) since the signal is differentially encoded as a phase difference (change) between successive data bits whilst XPM will in general be common to successive data bits. Since optical DQPSK does not require a phase-coherent local oscillator for demodulation this eliminates the need for adaptive polarization control. Since modulation and demodulation for optical DQPSK is functionally relatively straightforward it can readily be implemented using robust and compact electro-optic circuits such as phase modulators, optical couplers, splitters etc which can be readily integrated in the form of monolithic waveguide devices. The demodulator comprises an optical splitter for splitting the received DQPSK modulated optical signal into two parts which are each applied to a respective unbalanced Mach-Zehnder interferometer (MZI) fabricated in gallium arsenide or lithium niobate. A respective balanced optical to electrical converter is connected across the optical outputs of each MZI. Each MZI is unbalanced in that each has a time delay of approximately one symbol period (50 ps for a 20 Gsymbol/s line rate) of the data modulation rate, in one arm relative to that of the other arm, by making the physical length of one arm longer than that of the other arm. Each MZI is respectively set to impart a relative phase shift of π/4 and −π/4 by the application of an appropriate voltage to electrodes on either arm or a differential voltage. Other means of setting the MZIs can also be used.
However the operating point of the interferometer changes with time owing to transmitter and/or receiver characteristics. It is known to provide phase control to maintain the optimum operating point by controlling the path length difference in an interferometer-based optical DQPSK receiver. This control can be based on minimization of the power in the signal at the output of a differential detector. As the phase drifts away from the phase for maximum opening of the receiver “eye”, the received intensity tends to increase for two of the four data points and tends to decrease for the other two. On average there should be an increase in total RF power, and so the control can adjust the phase to minimize the RF power to maintain receiver eye opening at a maximum. However, the sensitivity of the control signal is low because the difference between maxima and minima is small and because the rate of change of the control signal is low or zero at the optimum operating point. Thus, the control signal is easily swamped by spurious noise and other effects. This gives a weak drive to the control loop so that the operating point is prone to error. No other simple method of setting the operating point is known.
The problem is exacerbated in optical systems, as opposed to radio DQPSK systems, because the carrier frequencies used are of the order of 104 times higher. Thus, to maintain the required phase difference between the two paths of the interferometer requires a highly sensitive control loop.